1. Field of the Invention
The present invention relates to a sublimation type thermosensitive image transfer recording medium, and a thermosensitive recording method using the thermosensitive image transfer recording medium.
2. Discussion of Background
Recently a demand for full color printers is increasing year by year. Representative recording methods for full color printers now available include the electrophotographic method, the ink-jet method, and the thermosensitive image transfer method. Of these methods, the thermosensitive image transfer method is most widely employed because of its advantages over the other methods in that maintenance is easy and operation is noiseless.
In the thermosensitive image transfer recording method, a solidified color ink sheet and a receiving sheet are employed, and a color ink is transferred imagewise from the ink sheet to the receiving sheet due to the thermal fusion of the ink or the sublimation of the ink, under the application of thermal energy by a laser beam or a thermal head which is controlled by electric signals.
Thus, the thermosensitive image transfer recording method can be roughly classified into two types, a thermal fusing image transfer type and a sublimation image transfer type. The sublimation image transfer type is advantageous over the thermal fusing type in that halftone can be obtained without difficulty and image gradation can be controlled as desired. These benefits exist because a sublimable dye is in principle sublimated in the form of independent molecules in such an amount as to correspond to the amount of thermal energy applied thereto, for instance, through a thermal head. Therefore, the sublimation image transfer type is considered the most suitable for color printers.
The sublimation image transfer recording method, however, has a shortcoming in that its running cost is high, because in this image transfer method, a yellow ink sheet, a magenta ink sheet, a cyan ink sheet, and when necessary, a black ink sheet, have to be employed in order to obtain a full-color image, with selective application of thermal energy to each ink sheet, and discarded after the recording, even though large unused portions remain on each ink sheet.
In order to eliminate this shortcoming, the following proposals have been made: (1) an equal speed mode in which an ink sheet and a receiving sheet are moved at the same speed for using the ink sheet in repetition and (2) an N-times use mode in which the running speed of the ink sheet is made lower than that of the receiving sheet so that the overlappingly used portions of the ink sheet at the first use and the second use are shifted little by little.
In the sublimation type thermosensitive image transfer recording method, the sublimation and evaporation reaction is fundamentally a reaction of zero order. Therefore, in the equal speed mode, the ink sheet cannot be used multiple times for printing because the printed image density significantly decreases as the number of printings increases, particularly in high image density areas, even though a sufficient amount of a dye for multiple printing is contained in the ink layer of the ink sheet.
In order to improve the drastic decrease in transferred image density during multiple printing, the inventors of the present invention proposed a sublimation type thermosensitive image transfer recording medium comprising a dye supplying layer and an image transfer facilitating layer in Japanese Laid-Open Patent Application 63-62866. In this recording medium, the sublimable dye discharging performance of the dye supplying layer is made greater than that of the image transfer facilitating layer.
However, the image density transferred from the above recording medium was still unsatisfactory, especially in high image density areas.